Liquid phase oxidation of an aromatic alkyl to an aromatic carboxylic acid is a highly exothermic chemical reaction. Volatilizable aqueous acidic solvents are used to contain the reaction mixture and to dissipate the heat of reaction. Conventionally, the oxidation of aromatic alkyls in the liquid phase to form aromatic carboxylic acids is generally performed in a vented, well-mixed oxidation reactor, with a substantial portion of the heat generated by the exothermic oxidation reaction being removed by evaporating directly from the reaction mixture a portion of the aqueous solvent and aromatic alkyl contained within the reactor.
The materials vaporized as a result of the heat generated in the exothermic reaction, together with unreacted oxygen and other aqueous components that may be present, pass upwardly through the reactor and are withdrawn from the reactor at a point above the reaction mixture liquid level for the reactor. The vapors are passed upwardly and out of the reactor to an overhead reflux condenser system where the vaporized solvent, water and aromatic alkyl are condensed. The resultant condensate is thereafter separated, e.g., in a reflux splitter, into a portion having a relatively higher water concentration and a portion having a relatively lower water concentration. The separated portion having a relatively lower water concentration, now at a temperature less than the reactor contents' temperature, is refluxed back into the reactor by gravity. Conventionally, the refluxed portion of the condensate is returned directly to the reactor through a process line external to the reactor. The non-condensable gases, carried along with the vaporized reactor material, are vented.
In operation, the reactor is fed by a liquid feed stream containing the aromatic alkyl, aqueous acidic solvent and an oxidation catalyst. An oxygen-containing gas is separately introduced into the reactor for oxidizing the aromatic alkyl to the aromatic carboxylic acid in the presence of the catalyst.
The reaction mixture contained in the reactor typically comprises a suspension of crystalline aromatic carboxylic acid in liquid, volatilizable, aqueous acidic solvent as mother liquor. The mother liquor contains, in addition to dissolved catalyst, some dissolved aromatic carboxylic acid product and lesser amounts of partially-converted species of such product. The mother liquor can also include a minor amount of unreacted, aromatic alkyl.
Aromatic carboxylic acid product quality is measured by optical density. At present, optical density of the obtained product limits the oxidation reactor operating temperature and pressure, as well as the reactor throughput and mother liquor recycle rate into the reactor. Because of the commercial importance of the oxidation of aromatic alkyls, however, it is highly desirable to improve the reactor conversion efficiency and quality of aromatic carboxylic acids produced by the oxidation of aromatic alkyls.
The invention disclosed herein tends to diminish so-called reactor "entrance" effects, thought to be caused by an oxygen deficiency at the point where the reactor feedstream feeds the reactor. The invention disclosed herein also tends to minimize color-body generation, known to limit aromatic carboxylic acid plant operating flexibility and capacity.